It's a discovery that could change our understanding of early humans. An incredibly well-preserved, 1.8-million-year-old skull from Dmanisi, Georgia suggests the evolutionary tree of the genus Homo may have fewer branches than previously believed.

A detailed analysis of the skull (above, in situ, and at left, fully excavated), appears in the latest issue of Science. In their report, the researchers – led by Georgian anthropologist David Lordkipanidze – write that it is "the world's first completely preserved hominid skull." And what a skull it is. It suggests that the earliest known members of the Homo genus (H. habilis, H. rudolfensis and H. erectus) may not have been distinct, coexisting species, at all. Instead, they may have been part of a single, evolving lineage that eventually gave rise to modern humans.

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Discovered alongside the remains of four other early human ancestors, all of which date to roughly 1.8-million years ago, "Skull 5," as the specimen is called, has a relatively tiny brain case and a protruding lower jaw – anatomical features reminiscent of Australopithecus, a more primitive human ancestor. And yet, its upper jaw resembles that of a 2.3-million-year-old specimen from Ethiopia, thought to have belonged to H. habilis; its bulky browline bears a strong resemblance to that of H. erectus; and its long, vertically oriented upper face and the overall shape of its braincase are unmistakably Homo. An anatomical chimera, Skull 5 possesses a surprising combination of features that have never before been observed together in an early Homo fossil.

Some fossils previously discovered at Dmanisi seemed to have links to H. erectus. But when the big lower jaw [of Skull 5] was found in 2000, some researchers suggested it belonged to a new species they called Homo georgicus.

With the discovery of the new, ﬁfth skull the researchers had to confront head-on the variation among all ﬁve. Age and sex probably account for much of it: The skulls are thought to have belonged to an elderly toothless male, two mature males, a young female, and an adolescent of unknown sex. This broad sample from one place and a short span of time is what makes Dmanisi an “exceptional site,” [says Tim White, a paleoanthropologist at UC Berkeley]. By analyzing the skull shapes with 3D computer-based methods, the researchers found that the range of variation in the group at Dmanisi was no greater than within living humans or chimps. The team concluded that all ﬁve skulls belong to a single, variable species.

In the end, the team settled on the cumbersome moniker of Homo erectus ergaster georgicus, which recognizes the skull as an earlier Georgian form of H. erectus.

But then, the remains of these five Dmanisi individuals are so very different. The skulls are so varied, in fact, that had they been found scattered throughout Africa, they could easily have been called separate species. This is the opinion of study co-author Marcia Ponce de Leon, who, together with University of Zurich neurobiologist Christoph Zollikofer, analyzed the shape and traits of the five Dmanisi skulls. The pair ultimately concluded that the Dmanisi specimens were every bit as diverse as African fossils traditionally pigeonholed into one of three different species, viz. H. erectus, H.habilis, and H. rudolfensis.

"It is [therefore] sensible to assume," Zollikofer said in a statement, "that there was a single Homo species at that time in Africa." He continues: "And since the Dmanisi hominids are so similar to the African ones, we further assume that they both represent the same species."

The claim stands to rewrite the evolutionary history of the human genus Homo by pruning three distinct offshoots of its evolutionary tree down to a single branch: Homo erectus. But this hypothesis has not been received lightly. Study co-author Philip Rightmire of Harvard University says the team's conclusion has set off a "bomb" in the field; and responses from experts around the world support this observation.

University College London's Fred Spoor criticized the team's methods of analysis in an interview with BBC News, noting that "they do a very general shape analysis of the cranium which describes the shape of the face and braincase in broad sweeping terms." Dean Falk, an anthropologist at Florida State University, supports the paper's conclusions. "I see no reason not to accept the authors' claim that the specimens all belong to one highly variable and highly sexually dimorphic species," she told Discovery News.

Still others insist that to argue over a single- vs. multiple-species hypothesis is to miss the study's most important takeaway entirely: that these Dmanisi hominins, with their relatively meager brain casings, are in fact the earliest evidence of Homo outside of Africa. "What's often missed in an announcement like this, when the focus becomes the skull or the names applied to the skull, it's the larger context," said White in an interview with the LA Times. "This is the very first evidence of the hominid expansion out of Africa."

"This is significant," he added. "I think that years, even decades from now, this will be seen as a classic turning point.